cnc

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NC and CNC machines and Control Programming
Introduction to NC and CNC machines CNC controls
and RS274 programming
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History of CNC
1949 US Air Force asks MIT to develop a
"numerically controlled" machine.
1952 Prototype NC machine demonstrated (punched
tape input)
1980- CNC machines (computer used to link
directly to controller)
1990- DNC external computer drip feeds control
programmer to machine tool controller
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Motivation and uses
To manufacture complex curved geometries in 2D or
3D was extremely expensive by mechanical means
(which usually would require complex jigs to
control the cutter motions) Machining
components with repeatable accuracy Unmanned
machining operations
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Advantages of CNC
- Easier to program - Easy storage of existing
programs - Easy to change a program - Avoids
human errors - NC machines are safer to operate -
Complex geometry is produced as cheaply as simple
ones - Usually generates closer tolerances than
manual machines
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Conventional milling machines
Vertical milling machine
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Conventional milling machines
Vertical Milling machine architecture
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Conventional milling machines
Horizontal Milling machine architecture
How does the table move along X- Y- and Z- axes ?
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NC machines
Motion control is done by servo-controlled
motors
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CNC terminology
BLU basic length unit ? smallest programmable
move of each axis.
Controller (Machine Control Unit, MCU)
? Electronic and computerized interface between
operator and m/c
Controller components 1. Data Processing Unit
(DPU) 2. Control-Loops Unit (CLU)
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Controller components
Data Processing Unit Input device RS-232 port/
Tape Reader/ Punched Tape Reader Data Reading
Circuits and Parity Checking Circuits Decoders
to distribute data to the axes controllers.
Control Loops Unit Interpolator to supply
machine-motion commands between data
points Position control loop hardware for each
axis of motion
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Types of CNC machines
Based on Motion Type Point-to-Point or Continuou
s path Based on Control Loops Open
loop or Closed loop Based on Power
Supply Electric or Hydraulic or Pneumatic Base
d on Positioning System Incremental or Absolute
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Open Loop vs. Closed Loop controls
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Open loop control of a Point-to-Point NC drilling
machine
NOTE this machine uses stepper motor control
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Components of Servo-motor controlled CNC
Motor
lead screw rotation
table moves
Motor speed control
position sensed by encoder
feedback
Two types of encoder configurations
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Motion Control and feedback
Encoder outputs electrical pulses (e.g. 500
pulses per revolution)
Rotation of the motor ? linear motion of the
table by the leadscrew
The pitch of the leadscrew horizontal distance
between successive threads
One thread in a screw ? single start screw Dist
moved in 1 rev pitch
Two threads in screw ? double start screw Dist
moved in 1 rev 2 pitch
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Example 1
A Stepping motor of 20 steps per revolution moves
a machine table through a leadscrew of 0.2 mm
pitch.
(a) What is the BLU of the system ?
(b) If the motor receives 2000 pulses per minute,
what is the linear velocity in inch/min ?
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Example 2
A DC servo-motor is coupled to a leadscrew (pitch
5mm) of a machine table. A digital encoder, which
emits 500 pulses per revolution, is mounted on
the leadscrew. If the motor rotates at 600 rpm,
find
(a) The linear velocity of the table
(b) The BLU of the machine
(c) The frequency of pulses emitted by the
encoder.
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Manual NC programming
Part program A computer program to specify
- Which tool should be loaded on the machine
spindle - What are the cutting conditions
(speed, feed, coolant ON/OFF etc) - The start
point and end point of a motion segment - how
to move the tool with respect to the machine.
Standard Part programming language RS 274-D
(Gerber, GN-code)
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History of CNC
The RS274-D is a word address format Each line
of program 1 block Each block is composed of
several instructions, or (words)
Sequence and format of words N3 G2 X1.4
Y1.4 Z1.4 I1.4 J1.4 K1.4 F3.2
S4 T4 M2
destination coordinates
dist to center of circle
tool
sequence no
spindle speed
feed rate
preparatory function
miscellaneous function
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Manual Part Programming Example
Tool size 0.25 inch, Feed rate 6 inch per
minute, Cutting speed 300 rpm, Tool start
position 2.0, 2.0 Programming in inches
Motion of tool p0 ? p1 ? p2 ? p3 ? p4 ? p5 ? p1
? p0
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1. Set up the programming parameters
Programming in inches
Use absolute coordinates
Feed in ipm
N010 G70 G90 G94 G97 M04
Spindle speed in rpm
Spindle CCW
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2. Set up the machining conditions
Machine moves in XY-plane
Use full-circle interpolation
Feed rate
Spindle speed
N020 G17 G75 F6.0 S300 T1001 M08
Tool no.
Flood coolant ON
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3. Move tool from p0 to p1 in straight line
Linear interpolation
target coordinates
N030 G01 X3.875 Y3.698
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4. Cut profile from p1 to p2
Linear interpolation
target coordinates
N040 G01 X3.875 Y9.125
or
N040 G01 Y9.125
X-coordinate does not change ? no need to program
it
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5. Cut profile from p2 to p3
Linear interpolation
target coordinates
N050 G01 X5.634 Y9.125
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6. Cut along circle from p3 to p4
circular interpolation, CCW motion
target coordinates
N060 G03 X7.366 Y9.125 I6.5 J9.0
coordinates of center of circle
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7. Cut from p4 to p5
Linear interpolation
target coordinates (Y is unchanged)
N070 G01 X9.302
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8. Cut from p5 to p1
Linear interpolation
target coordinates (see step 3)
N080 G01 X3.875 Y3.698
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9. Return to home position, stop program
Linear interpolation
target coordinates (see step 3)
N090 G01 X2.0 Y2.0 M30
end of data
N100 M00
program stop
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Automatic Part Programming
Software programs can automatic generation of CNC
data
Define Tool
CNC data
Make 3D model
Simulate cutting
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Automatic part programming and DNC
Very complex part shapes ? very large NC program
NC controller memory may not handle HUGE part
program
computer feeds few blocks of NC program to
controller
When almost all blocks executed, controller
requests more blocks
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Summary
CNC machines allow precise and repeatable control
in machining
CNC lathes, Milling machines, etc. are all
controlled by NC programs
NC programs can be generated manually,
automatically
Additional references RS274D code descriptions
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